This invention relates generally to electrical systems for vehicles of the type which includes a primary battery to provide electrical power for the purpose of starting and operating the vehicle engine and an auxiliary battery to provide voltage sufficient for the operation of electrical accessories of the vehicle. More particularly, the invention provides an auxiliary battery monitor capable of effecting an alarm signal to the operator of the vehicle indicating a low voltage condition of the auxiliary battery in the event that a failure of any component in the electrical system occurs and results in one or more of the accessories failing to operate if the vehicle is driven subsequent to an overnight rest stop or similar period of inactivity.
Many of the electrical accessories employed in vehicles, particularly of the tractor/trailer type, often impose extraordinary electrical loads on the normal power supplies provided with this type of vehicle, especially when the engine is not operating. Such a load may well result in battery stress which in turn may cause difficulty in starting the engine and difficulty in charging the battery sufficiently once the engine is operating. In view of the extraordinary load upon the normal battery caused by the operation of the electrical accessories, such as the radio, dome lamps, tail lamps, other communications equipment such as CB units, auxiliary power supplies have been provided, such as an auxiliary battery, to supplement the primary battery and often exclusively to meet the electrical load constituted by these accessories. The auxiliary power supplies often are the exclusive power source for the electrical accessories, especially while the engine is inactive such as would occur during an overnight rest stop or similar occurrence.
Failures occur in various components of the electrical system, such as the charging devices, including the voltage regulator means and/or the alternator, where provided, or upon failure of the electrically operated solenoid switch means which may disconnect the charging devices from the batteries. One result is draining the charge from the auxiliary battery sufficiently that accessories such as the tail lamps may fail, even subsequent to the starting and operation of the engine. If the auxiliary battery output voltage is reduced below the output voltage required to operate one or more of the accessories, the primary battery may nevertheless have sufficient charge so that the engine can be started and operated to enable the vehicle to be driven after such layover. The voltage output of the auxiliary battery could nevertheless be incapable of supporting the operation of the tail lamps, for example. Under such conditions, the operator of the vehicle could start the engine and proceed along the road without having knowledge that accessories such as the tail lamps may not be operational.